1. Field of the Invention
The present invention relates to a positional change measurement device, a positional change measurement method, and an image forming apparatus.
2. Description of the Related Art
Measurement of the traveling velocity or traveled distance of a surface of a belt or the like moving rotationally is performed in various techniques.
For example, a so-called tandem system has become the mainstream system employed by a color image forming apparatus using an electrophotographic process, in order to meet demands for high-speed image formation.
In a color image forming apparatus using the tandem system, four photosensitive bodies are arranged in parallel, the photosensitive bodies corresponding to toner of respective four colors (black, cyan, magenta, and yellow).
Then, toner images of the respective colors are formed on the photosensitive bodies and finally superimposed on one another on a recording medium such as a sheet of paper (a standard-sized sheet, a postcard, a cardboard, an OHP sheet, or the like) to obtain a color image.
There are two methods used in this event: a direct transfer method in which the toner images of the respective colors are superimposed directly on the recording medium, and an intermediate transfer belt method.
In the intermediate transfer belt method, the toner images of the respective colors on the photosensitive bodies are superimposed on and transferred to an intermediate transfer belt to form a color image, and collectively transferred to a recording medium.
In such a transfer, color misalignment occurs unless a conveyer belt for conveying the recording medium in the case of the direct transfer method or the intermediate transfer belt in the case of the intermediate transfer belt method is moved with high accuracy.
Meanwhile, inkjet is known as a type of a color image forming apparatus. The inkjet-type color image forming apparatus obtains a color image by combining ink images of multiple colors and forms the color image on a recording medium such as paper moving in a certain direction.
Such an inkjet-type color image forming apparatus also suffers from color misalignment unless the movement of the recording medium is controlled with high accuracy.
Thus, also in the image forming apparatus, the traveled distance or traveling velocity of the recording medium, the conveyer belt, or the intermediate transfer belt needs to be measured with high accuracy in order to control the movement thereof accurately.
It has been proposed to use speckle patterns for control of driving of a recording medium, a conveyer belt, an intermediate transfer belt, or photosensitive bodies in an image forming apparatus (Japanese Patent Application Publication Nos. 2009−15240, 2003−267591, and 2010−55064 (referred to as Patent Documents 1 to 3, respectively, hereinbelow)).
Speckle patterns are random patterns produced when coherent light beams, such as laser light, are applied to a surface having minute asperities, are reflected by or penetrate the surface, and interfere with each other.
It is known that, when the surface irradiated with the coherent light moves in a planar direction, because an illuminated area is moved and obtained speckle patterns are changed bit by bit, a cross-correlation peak position of an image of the speckle patterns at around the movement is moving.
Hence, the positional change (traveled distance or traveling velocity) of the surface can be measured through measurement of the positional change of the speckle patterns.
Herein, a target to be measured for its positional change in the planar direction by use of the speckle patterns is called a dynamic measured surface.
Patent Documents 1 to 3 each disclose measurement of the positional change of the dynamic measured surface by use of speckle patterns, but do not consider influence of fluctuations in a measurement environment temperature.
The measurement environment temperature is a temperature affecting measurement of the positional change of the dynamic measured surface.
Fluctuations in the measurement environment temperature affect measurement of the positional change of the dynamic measured surface, and cannot be ignored when accurate measurement is demanded.